This application claims priority to an application entitled xe2x80x9cApparatus and Method for detecting a Received On-Off Keying Signal in a CDMA Mobile Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Sep. 6, 1999 and assigned Ser. No. 99-37639, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to an apparatus and method for detecting a received signal in a CDMA mobile communication system, and in particular, to an apparatus and method for detecting a received on-off keying modulated signal.
2. Description of the Related Art
In general, an existing IS-95 CDMA (Code Division Multiple Access) mobile communication system provides a voice service. However, future IMT-2000 compatible CDMA mobile communication system will provide a high-speed data service in addition to the voice services. The IMT-2000 standard aims to provide a high-quality voice service, a moving picture service, and an Internet search service.
In addition, the IMT-2000 standard provides for a new paging indicator signal using on-off keying modulation. In the common CDMA mobile communication system, a mobile station receives a paging channel from a base station and accesses its unique slot out of a plurality of slots constituting the paging channel. However, in the case where the base station provides the paging indicator signal as stated above, the mobile station demodulates its unique paging slot out of the paging slots constituting the paging channel, upon receipt of the paging indicator signal. Therefore, when the paging indicator signal is provided, the mobile station can demodulate its unique slot only when the paging indicator signal is received, rather than demodulating its unique slot whenever the paging channel is received, thereby making it possible to reduce power consumption of the mobile station. Meanwhile, for the paging indicator signal, an on-off keying signal is typically used.
FIG. 1 illustrates an example of a common demodulation device in a CDMA mobile communication system which uses a paging indicator signal. FIG. 1 shows a demodulation device wherein a transmitter is synchronized with a receiver. Such demodulation is referred to xe2x80x9ccoherent detectionxe2x80x9d.
Referring to FIG. 1, a low pass filter (LPF) 110 filters a received signal in a corresponding frequency band. A sampler 120 extracts a symbol from an output of the low pass filter 110 in sync with a transmitter (not shown). A symbol detector 130 detects a symbol by comparing a value of the symbol extracted by the sampler 120 with a threshold value for symbol detection. Here, a fixed threshold value is used for the symbol detection threshold value.
FIG. 2 illustrates another example of a common demodulation device in a CDMA mobile communication system, which uses a paging indicator signal. Specifically, FIG. 2 shows a demodulation device for the case where the transmitter is not exactly synchronized with the receiver. Such demodulation is referred to xe2x80x9cnon-coherent detectionxe2x80x9d.
Referring to FIG. 2, a low pass filter 210 filters a received signal in a corresponding frequency band. An envelope detector 220 extracts an envelope of the low-pass-filtered signal. A sampler 230 extracts a symbol from an output of the envelope detector 220. A symbol detector 240 detects a symbol by comparing the symbol extracted by the sampler 230 with a threshold value for symbol detection. Here, a fixed threshold value is used for the symbol detection threshold value.
The demodulators of FIGS. 1 and 2 are useful where there exists a rare variation of channel conditions and there exist only Gaussian noises, or where an on/off ratio is relatively high. That is, the demodulators employ threshold-dependent demodulation, without considering the variation of the channel conditions. Therefore, the conventional demodulators are not effective in real mobile environments where the channel conditions vary greatly and/or the on/off ratio is not so high, such as in the fading environments.
In the mobile communication environments, a received signal of the mobile station can be modeled as shown in Equation (1) below.
s(t)=Ac(t)cos xcfx89ctxc2x7(Ri(t)+jRq(t))+N(t)xe2x80x83xe2x80x83(1)
where s(t) indicates a signal received at the mobile station, Ac(t) indicates a transmitted original signal, Ri(t) and Rq(t) indicate phase variation of the channel, and N(t) indicate the Gaussian noise components.
In Equation (1), if the receiver can exactly perceive the phase variation of the channel, the received signal can be represented by Equation (2) below.
xe2x80x83R(t)=ni(t) (if s0 is sent)
                              R          ⁡                      (            t            )                          =                                                            [                                                                                                    R                        i                                            ⁡                                              (                        t                        )                                                              2                                    +                                                                                    R                        q                                            ⁡                                              (                        t                        )                                                              2                                                  ]                                      ·                          A              c                                +                                                    n                i                            ⁡                              (                t                )                                      ⁢                          xe2x80x83                        ⁢                          (                              if                ⁢                                  xe2x80x83                                ⁢                s1                ⁢                                  xe2x80x83                                ⁢                is                ⁢                                  xe2x80x83                                ⁢                sent                            )                                                          (        2        )            
In Equation (2), transmission of s0 means that no signal is transmitted, and transmission of s1 means that a signal is transmitted. Further,             [                                                  R              i                        ⁡                          (              t              )                                2                +                                            R              q                        ⁡                          (              t              )                                2                    ]        ·      A    c  
indicates an amplitude of the received signal,       [                                        R            i                    ⁡                      (            t            )                          2            +                                    R            q                    ⁡                      (            t            )                          2              ]  
indicates phase variation components of the channel, and ni(t) indicates the in-phase components of the Gaussian noise.
That is, it is noted from Equation (2) that the amplitude of a signal varies greatly as compared with the noises, in the real mobile environments where channel conditions vary violently and the on/off ratio is not so high, such as in the fading environments. Therefore, the conventional demodulation method, which depends on the fixed threshold value, cannot accurately detect a symbol from the received signal.
It is, therefore, an object of the present invention to provide an apparatus and method for detecting a received on-off keying signal depending on a variable threshold value in a CDMA mobile communication system.
It is another object of the present invention to provide an apparatus and method for detecting a received on-off keying signal depending on a variable threshold value according to channel environments in a CDMA mobile communication system.
It is further another object of the present invention to provide an apparatus and method for detecting a received on-off keying signal depending on a variable threshold value, by using a reference signal which experiences the same channel conditions as an on-off keying signal to be demodulated, in a CDMA mobile communication system.
It is yet another object of the present invention to provide an apparatus and method for detecting a received on-off keying signal depending on a variable threshold value, by using a reference signal for estimating the channel conditions that the on-off keying modulated signal experiences, in a CDMA mobile communication system.
It is still another object of the present invention to provide an apparatus and method for detecting a received on-off keying signal depending on a variable threshold value by using the power value of a pilot channel signal for estimating the channel conditions that the on-off keying modulated signal experiences, in a CDMA mobile communication system.
It is still another object of the present invention to provide an apparatus and method for detecting a received on-off keying signal depending on a variable threshold value, by using the power of a common channel signal for estimating the channel conditions that the on-off keying modulated signal experiences, in a CDMA mobile communication system.
It is still another object of the present invention to provide an apparatus and method for detecting a received on-off keying signal depending on a variable threshold value, by using a channel signal, a power value of which can be detected in a mobile station, in a CDMA mobile communication system.
It is still another object of the present invention to provide an apparatus and method for detecting a received on-off keying signal depending on a variable threshold value, by calculating the power of a received on-off keying signal after channel passing, determining a signal detection threshold value using a power level of the channel-passed reference signal, and detecting a received on-off keying signal according to the signal detection threshold value, in a CDMA mobile communication system.
To achieve the above and other objects, an apparatus is provided for detecting an on-off keying signal in a CDMA mobile station device. A first calculator receives an on-off keying signal and a reference signal transmitted from a base station transmitter at a given transmission power level, despreads the received on-off keying signal, and generates a symbol power level of the despread on-off keying signal. A second calculator despreads the reference signal and generates a power level of the despread reference signal. A multiplier generates a variable threshold value by multiplying the power level of the despread reference signal by a gain determined by a ratio of the transmission power of the reference signal from the base station transmitter to the transmission power of the on-off keying signal from the base station transmitter. A comparator compares the variable threshold value with the symbol power level of the despread on-off keying signal. Further, a symbol detector detects the symbol of the on-off keying signal by comparing a comparison value from the comparator with a prescribed threshold value.